The present invention relates to a magnetooptical recording medium allowing information recording and reproduction by irradiation with a laser beam, utilizing a magneto-optic effect, and methods for information recording and reproduction using the magnetooptical recording medium.
In recent years, a magnetooptical recording medium has become a subject of attention in the field of a rewritable recording method of high recording density. In such a recording method, information or data is recorded in the recording medium by forming a magnetic domain in a magnetic film of the medium by means of thermal energy of a laser beam from a semiconductor laser, and information is read out from the medium, utilizing magneto-optic effect. The above-noted trend is based on need for a larger amount of recording capacity to be achieved by higher recording density of such a recording medium.
By the way, linear density of an optical disc, such as a magnetooptical recording medium, is largely dependent on the wavelength of a laser beam and the numerical aperture of an objective lens used in an optical system for reproducing information. When the wavelength xcex of a laser beam used in a reproducing optical system and the numerical aperture NA of an objective lens are determined, a bit or pit periodicity or pitch is defined as xcex/2NA which is a minimum scale or limit of detection.
Track density of the optical disc is, on the other hand, chiefly limited by crosstalk. The crosstalk is largely dependent on a laser beam distribution or profile on a medium surface and is expressed as a function of xcex/2NA, similar to the bit periodicity mentioned above. Thus, the wavelength of a laser beam must be shortened and the numerical aperture of an objective must be enlarged in order to increase recording density of the conventional optical disc.
However, there are limitations to improvement of the wavelength of a laser light and the numerical aperture of an objective. Techniques therefor have been developed, which improve the structure of a recording medium and a method of reading out data bits so that the linear recording density can be improved.
For example, Japanese Patent Laid-open No. 3-93058 discloses a method for improving the recording density of an optical disc. The process is performed in the following sequence. First, a recording medium, which comprises a readout layer and a recording layer, is prepared. After the direction of magnetization in the readout layer is oriented in a single direction prior to information reproduction, then information held in the recording layer is transferred to the readout layer. Thus, it becomes possible to reduce interference between codes at the time of information reproduction and to reproduce information recorded at a pitch below the diffraction limit of light.
However, the magnetooptical reproducing method of Japanese Patent Laid-open No. 3-93058 requires a step of aligning the magnetization direction of the readout layer in a predetermined direction, which is conducted before projection of a laser beam onto the readout layer. Thus, it is necessary to add a magnet for initializing the readout layer to a conventional apparatus. Due to such addition, problems arise, such as more complicated structure of a magnetooptical recording apparatus, difficulty of down-sizing, and higher cost of an apparatus.
It is an object of the present invention to provide a magnetooptical recording medium which enables realization of a compact apparatus and a high S/N reproduction of information recorded at a pitch below the diffraction limit of light and methods for recording information on the magnetooptical recording medium and reproducing information therefrom.
The object is attained by a magnetooptical recording medium comprising a first magnetic layer which is composed of a perpendicularly magnetizable film at both room temperature and a raised temperature and in which information is to be stored, and a second magnetic layer which is an in-plane magnetizable film at room temperature and becomes a perpendicularly magnetizable film at the raised temperature.
Further, the object is attained by a magnetooptical recording medium comprising a first magnetic layer which is a perpendicularly magnetizable film at both room temperature and a raised temperature and in which information is to be stored, and a second magnetic layer which is a perpendicularly magnetizable film at room temperature and becomes an in-plane magnetizable film at the raised temperature.
Further, the object is attained by a method of reproducing information from a magnetooptical recording medium comprising a first magnetic layer which is a perpendicularly magnetizable film at both room temperature and a raised temperature and in which information is to be stored, and a second magnetic layer which is a perpendicularly magnetizable film at room temperature and becomes an in-plane magnetizable film at the raised temperature. The method comprises a step of projecting a light spot on the recording medium from the side of the second magnetic layer, a step of changing only a high-temperature area within a light spot projection portion of the second magnetic layer to an in-plane magnetization film, and a step of detecting information stored in the first magnetic layer by detecting a reflective light from the recording medium, which is influenced by magneto-optic effect of the perpendicular magnetization film portion in an area other than the high-temperature area of the light spot projection portion.
Further, the object is attained by a method of reproducing information from a magnetooptical recording medium comprising a first magnetic layer which is a perpendicular magnetization film at both room temperature and raised temperatures and in which information is to be stored, and a second magnetic layer which is an in-plane magnetization film at room temperature and changed to a perpendicular magnetization film at raised temperatures. The method comprises a step of projecting a light spot on the recording medium from the side of the second magnetic layer, a step of changing only a high-temperature area within a light spot projection portion of the second magnetic layer to a perpendicular magnetization film and transferring information stores in the first magnetic layer to a perpendicular magnetization film portion of the second magnetic layer, and a step of detecting information transferred to the second magnetic layer by a reflective light from the recording medium, which is influenced by magneto-optic effect of the perpendicular magnetization film portion of the second magnetic layer.
Further, the object is attained by a method of reproducing information from a magnetooptical recording medium comprising a first magnetic layer which is a perpendicular magnetization film at both room temperature and raised temperatures and in which information is to be stored, and a second magnetic layer which is an in-plane magnetization film at room temperature and changed to a perpendicular magnetization film at raised temperatures. The method comprises a step of projecting a light spot on the recording medium, a step of applying a magnetic field, whose direction is modulated in accordance with information, to a light spot projection portion of the recording medium and orienting the magnetization direction of the second magnetic layer to the direction of the applied magnetic field, and a step of transferring the magnetization of the second magnetic layer to the first magnetic layer to form record bits.
These advantages and others will be more readily understood in connection with the following detailed description of the preferred embodiments taken in conjunction with the drawings.